This invention relates to a device for generating mechanical oscillations in an examination object using magnetic resonance elastography (MRE) according to the generic part of claim 1 and to a process for the magnetic resonant elastographic determination of biomechanical properties of tissue according to claim 23.
By means of magnetic resonance elastography (MRE) it is possible to detect biomechanical properties of biological tissues. This method is comparable with “instrumental palpatory findings”, with which it is possible, however, to quantify the biomechanical properties not only of subsurface, but also of deep-seated tissues or organs with good transillumination and resolution. Comparable to classical palpation, but without limitation to subsurface tissues and objectively measurable, information on pathological changes of tissues and organs thus is accessible. MRE is based on the principles of magnetic resonance tomography (MRT). In the case of MRE, however, periodic particle displacements in the tissue to be examined must be generated in addition by means of mechanical excitation. The detection of particle motion, however, is effected by magnetic preparation of the nuclear spins and by motion-sensitive phase contrast techniques.
The resulting gray-scale images have a typical wave character, which represents the periodic tissue distortions (shear waves) caused by the mechanical excitation. From the distortions detected, elastic characteristics of the tissue such as shear modulus, Young's modulus, compression modulus or Poisson ratio can be calculated. Measuring the distortion in all spatial directions allows the complete quantification of the elastic characteristics in consideration of their directional dependence.
Examinations of the frequency and amplitude dependence of the periodic tissue distortions thus provide information on viscoelastic properties of tissues. Non-linear stress-distortion relationships, which likewise have a high potential for characterizing tissue properties, are accessible by measuring harmonic oscillations of the tissue distortion, which are present along with the excitation frequency.
The number of observable shear waves in the tissue to be examined depends on the elastic properties thereof and on the frequency of the mechanical excitation. So far, three different types of mechanical excitation units have been used in MRE, in order to generate displacements in tissues, including:    1) excitation units which are based on the linear expansion of piezoelectric crystals,    2) excitation units which are based on a motion reversal of current-carrying coils moving in the magnetic field of the tomograph (electromechanical excitation),    3) passive, pressure-activated excitation units which are driven via pneumatic lines.
The known devices have a number of limitations and disadvantages in connection with the previously available excitation methods. For instance:    1) Piezoelectric methods:            High voltage: Piezoelectric crystals are operated with high-voltage amplifiers with a voltage of up to 1 kV. This is problematic for use in patient examinations and involves an enormous safety effort.        Complex mechanics: Since the displacement amplitudes also of piezostacks with a length of 200 mm lie in the range of 200 μm, reversals must be realized by means of levers, in order to provide for displacements in the order of 1 mm.        Phantom images: Due to lever reversal, springs, and an aluminum sleeve with a length of about 250 mm, in which the piezocrystals are biased, problems arise in connection with phantom images. The same are caused by the various metal parts necessary for construction, which lead to distortions of the magnetic field and hence to phantom images, the extent of which partly renders the evaluation of the image material impossible.        Positioning: Due to the length of the finished excitation unit of usually 250 mm and a maximum diameter of the tomograph of 60 cm, positioning the excitation unit in part only is possible to a restricted extent depending on the examination object.            2) Electromechanical excitation units:            Positionability: The main disadvantage of the electromechanical excitation units is the limitation to specified coil orientations in the magnetic field, as otherwise the magnetic fields necessary for the motion cannot be induced by the alternating voltage applied. As a result, applicability either is restricted or complicated mechanical reversal mechanisms become necessary.        Phantom images: The use of aluminum for an improved dissipation of heat and enameled copper wire in turn leads to image extinctions, for whose prevention the coil of the mechanical excitation unit must be located at least 2 coil diameters away from the examination object. The magnetic fields induced therein additionally cause phantom images in dependence on the alternating voltage increasing with the displacement. This often prevents the generation of larger displacement amplitudes advantageous for MRE image material to be evaluated reliably.        Phantom images caused by power supply cables: For the power and voltage supply of electromechanical excitation units, corresponding cables are required, which reach into the center of the magnet of the tomograph. Although shielded cables are used, which lead over a filter plate into the examination room, electromagnetic interferences, which are trapped by the cables and in turn lead to image artifacts, cannot always be excluded.        For the different organs/tissues to be examined special excitation units must be developed, which are optimally adapted to the respective requirements.        
In MRE, the mechanical tissue oscillations (shear waves) induced are detected by means of magnetic resonance tomography (MRT). For this purpose, the motions of the particles in the tissue must be encoded magnetically, which according to the prior art is effected by means of synchronously oscillating magnetic field gradients, i.e. the acoustic excitation for generating the oscillations and the magnetic encoding are effected with identical frequencies.
Oscillating motion encoding gradients (MEG) can be included in almost any MRT imaging technique. Therefore, MRE imaging techniques now are available, which are based on the principles of spin-echo or gradient-echo techniques. Particularly interesting imaging techniques include fast EPI (echo planar imaging) or SSFP (steady-state free precession) imaging techniques.